Move the post-processing of the QEMU_CAPS_CHARDEV_FD_PASS flag to the
new function.

The clearing of the capability is based on the presence of
VIR_QEMU_PROCESS_START_STANDALONE so we must also pass in the process
start flags.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>